Selective surface metallization, approaches

The chemistry of the selective-surface metallization process has taken two paths. One approach (53) involves photo-induced local modifications in... 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

The immohilization of the cage complexes on the surface through apical groups offers interesting application possibilities. This approach enables one to obtain ion-exchange resins especially selective for metal ions. The immobilized optically active cations allow one to obtain ion-exchange resins for the separation of optical isomers, such as racemic amino acids or optically active complexes. 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

X.-B. Long, M. Miro, E.H. Hansen, Universal approach for selective trace metal determinations via sequential injection-bead injection-Lab-on-Valve using renewable hydrophobic bead surfaces as reagent carriers, Anal. Chem. 77 (2005) 6032. 

The presence of a liquid metal phase makes it difficult for one to study and utilize the effect of liquid metal embrittlement because of the need to employ high temperatures, the difficulties in ensuring good wetting conditions, and especially the need to remove the residual films and traces of the active component. A principally new approach to overcoming these issues was the use of liquid metal embrittlement in the absence of a liquid metal phase. This is achieved in an electrochemical cell by the cathode reduction of ions of the selected surface-active metal directly at the sample surface. The amount of metal formed can be controlled and fine-tuned down to the formation of monolayers. It is also possible for one to conduct these studies at room temperature [59,107,108] (Figures 7.51 and 7.52). 

The effects of pulsed waveforms are extremely complex and poorly understood, but the following effects are generally accepted. During the off period of a pulse, no net electron transfer can take place and the cathode surface is refreshed with metal cations as a result of convective diffusion. During the on period, the surface metal ion concentration will initially approach the bulk solution value but will decay with time, i.e. the technique involves non-steady state diffusion. A limiting case is a surface metal ion concentration of zero, i.e. complete mass transport control. The reverse (anodic) current may lead to selective dissolution of high points on the deposit due to their enhanced current density, producing a more compact or smooth surface. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

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